Our main research efforts are directed toward a better understanding of the structural and functional properties of RNAs, the enzymes that modify these molecules, and the involvement of GTP in protein biosynthesis. Our experimental approach involves the use of analogs of nucleic acid components. These are generally incorporated into the RNAs, the RNAs isolated, and their properties studied. The uracil analog 5-florouracil (FU) is incorporated into cellular tRNAs in place of uracil and results in the synthesis of FU-containing tRNAs deficient in a number of modified nucleosides normally derived from uridine. We are examining a number of the biological, chemical, and physical properties of these molecules. Photoaffinity analogs of nucleic acid components have recently been employed in a number of biological systems and promise to be powerful biochemical probes. We are using 8-azido-GDP and 8-azido-GTP to examine the mode of action of GTP in protein biosynthesis. In addition, 8-azido-adenosine derivatives are being used in an attempt to generate extremely light-sensitive macromolecules. These would then be used in studying a variety of macromolecular complexes.